1. Field of the Invention
The present invention relates to a physical quantity detecting apparatus and, more particularly, to a physical quantity detecting apparatus for measuring a physical quantity (for example, transmitted torque) of an object of measurement.
2. Description of the Related Art
In various kinds of rotating drive mechanisms there is a demand for simple and accurate measurement of a physical quantity such as a transmitted torque because such measurement is exceedingly useful in a diverse range of industrial field for analyzing drive mechanisms and obtaining a better understanding of their operating condition.
Rotary drive mechanisms are used as prime movers in virtually every sector of industry, the most common types thereof being automobile engines, electric motors of electric cars and industrial motors.
In accurately obtaining and analyzing the operating condition of such mechanisms, it is as important to be able to accurately determine a transmitted torque as it is to determine the number of revolutions. Measurement of a torque is particularly important in the case of automobile engines because by measuring the torque at the engine and at the transmission, propeller shaft, differential gear and other points of the drive system it is possible to control the ignition timing for the engine, the amount of fuel injection, the timing for transmission shift, the gear ratio, etc. As a result of the optimum control of these factors, it is possible to improve fuel efficiency, driving characteristics, etc.
In the case of industrial motors, accurate torque measurement can also provide data for optimizing control and diagnosis of rotary drive systems, thereby improving energy efficiency and driving characteristics.
For these purposes, various kinds of torque detecting apparatus have conventionally been proposed, one of them being an apparatus for noncontactingly measuring the torque transmitted through a rotary magnetic material as magnetostriction.
When a torque is transmitted through a rotary drive system, strain is produced in the rotary members such as a rotating shaft and a clutch disc in proportion to the transmitted torque. Therefore, it is possible to measure the transmitted torque noncontactingly by detecting the magnetostriction of a rotary magnetic material which transmits the torque by using a magnetic sensor.
FIGS. 7 and 8 show an example of a torque transmission mechanism of a vehicle engine provided with a magnetic sensor 12 of a torque detecting apparatus. FIG. 7 is a schematic side elevational view of the magnetic sensor 12, and FIG. 8 is a schematic sectional view of the magnetic sensor 12 shown in FIG. 7, taken along the line VIII--VIII.
As is known, the torque produced in the engine is transmitted to a rotary flywheel (not shown) through a torque transmitting shaft 10, and is transmitted to the transmission through a clutch disc which comes into frictional contact with the flywheel.
When a torque is transmitted in this manner, anisotropy of strain .epsilon. which is proportional to the magnitude of the transmitted torque is generated on the torque transmitting shaft 10 and the rotary discs such as the clutch disc and the flywheel. If the torque transmission member is made of a ferromagnetic material, it is possible to measure the transmitted engine torque by magnetically detecting the magnitude of the anisotropy of strain e generated by utilizing the magnetostrictive effect.
In the torque measuring apparatus, therefore, in order to make the rotary member to which a torque is transmitted a rotary magnetic material, the torque transmitting shaft 10 or the flywheel themselves is made of a ferromagnetic material, or a ferromagnetic material is attached to the surface of the torque transmitting shaft 10 or the flywheel, and the magnetic sensor 12 is opposed to the rotary magnetic material formed in this manner with a predetermined space therebetween.
The magnetic sensor 12 is composed of a U-shaped excitation core 14 which is disposed in parallel to the torque transmitting shaft 10, and a U-shaped detection core 18 which is disposed inside the excitation core 14 such as to be orthogonal thereto, with an excitation coil 16 wound around the excitation core 14, and a detection coil 20 wound around the detection core 18.
FIG. 10 is a block diagram of the torque detecting apparatus. To the excitation coil 16 a sine-wave voltage is applied from an AC power source 22 for alternating magnetization of the torque transmitting shaft 10, which is opposed to the magnetic sensor 12. When a torque is transmitted through the torque transmitting shaft 10, stress is produced in the torque transmitting shaft 10 and a magnetic flux component is generated in the direction orthogonal to the direction of excitation by virtue of the magnetostrictive effect. The magnetic flux component is detected by the detection coil 20 as an induced voltage. The induced voltage is amplified by an alternating amplifier 24 and is thereafter rectified by a detector 26. This rectified signal S (hereinunder referred to as "torque detection signal") is output as a torque detection signal.
The torque detection signal S is output as the sum of the component which depends upon the transmitted torque and an offset component which does not depend upon the transmitted torque. It is therefore necessary to subtract the offset component from the torque detection signal.
The magnitude of the offset component irregularly varies with the rotation of the rotary magnetic material, although the transmitted torque is zero, as shown in FIG. 12. Therefore, a technique of subtracting the offset component at each position of the rotary magnetic material is necessary for detecting the torque with high accuracy.
Japanese Patent Laid-Open Patent Publication Nos. 55533/1987 and 55534/1987 disclose apparatuses using such a technique. This apparatus measures the torque transmitted through a rotary magnetic material and having a plurality of rotational angle positions as torque inflection points at each interval between the torque inflection points. This apparatus is characterized in that an offset signal which is output from a magnetic material in dependence on the rotational angle of the rotary magnetic material is preset in each interval between the torque inflection points and the offset signal is subtracted from the detection signal output from the magnetic sensor on the basis of a timing signal which represents the rotational angle of the rotary magnetic material and the torque inflection point, thereby outputting the average torque value in each interval between the torque inflection points. In this way, it is possible to measure the torque transmitted through the rotary magnetic material in each interval between the torque inflection points without being influenced by the offset component.
This torque detecting apparatus however has the following two problems.
(a) In this apparatus, although fluctuation of the offset component with the rotation of a rotary magnetic material is taken into consideration, no attention is paid on the output which depends on the torque, namely, fluctuation of the torque detection sensitivity. Therefore, there is a limitation in enhancing the accuracy in torque detection.
More specifically, in this kind of torque detecting apparatus, since the torque is detected by utilizing a change in the magnetic characteristics produced on the surface of a rotary magnetic material, the detecting accuracy greatly depends upon the nonuniformity of the magnetic characteristics of an object of measurement. If the magnetic characteristics distribute nonuniformly in an object of measurement, namely, rotary magnetic material, the torque detection output (sensor output) disadvantageously fluctuates with the rotation of the shaft, as shown in FIG. 13, although the applied torque is constant.
The present inventors have investigated on the cause of such fluctuation of the torque detection output Torque detection output S is represented by the function of the applied torque Tq as follows: EQU S=Sens.multidot.Tq+Offs (1)
wherein Sens represents a sensitivity and Offs an offset output. The sensitivity means the increment in the detection output per unit torque, and the offset output means the sensor output obtained when the applied torque is zero.
As a result of investigation, the present inventors have confirmed that the sensitivity and the offset output shown in the equation (1) fluctuate due to the nonuniformity of the magnetic characteristic in the object of measurement.
This is represented by the following equation: EQU S(P)=Sens(P).multidot.Tq+Offs(P) (2)
wherein P represents the rotational position (location of measurement) of the rotary magnetic material.
The sensitivity Sens(P) and the offset signal Offs(P) vary as shown in FIGS. 11 and 12, respectively. It goes without saying that when the rotary magnetic material rotates, the torque detection output S(P) fluctuates as shown in FIG. 13, even if the applied torque is constant
It is therefore obvious that if the fluctuation of the offset component Offs(P) produced with the rotation of the rotary magnetic material is merely considered without taking the output depending upon the torque, namely, the torque detection sensitivity Sens(P) into any consideration, it is impossible to measure the transmitted torque Tq with high accuracy.
The following may be regarded as the cause of the nonuniformity of the magnetic characteristics in the object of measurement:
1 nonuniformity of composition, PA0 2. nonuniformity of structure, and PA0 3. distribution of residual stress.
Accordingly, if a rotary magnetic material having uniform composition and structure and no residual distribution is produced by improving the manufacturing process of an object of measurement, the above-described problems are solved. Actually, however, it is impossible because it requires complete control of the manufacturing process.
(b) In the torque detecting apparatus, since fluctuation of the torque detection signal in accordance with the temperature characteristic is not taken into consideration, there is also limitation in enhancing the detecting accuracy.
As a result of investigation, the present inventors have confirmed that the sensitivity and the offset output shown in the equation (1) fluctuates with a change in temperature of the detecting apparatus. FIGS. 14 and 15 show an example of the temperature dependence of the sensitivity Sens(T) and the offset output Offs(T), respectively. This will be represented by the following equation: EQU S(T)=Sens(T).multidot.Tq+Offs(T) (3)
wherein T represents a temperature of the torque detecting apparatus.
The fluctuation is considered to be caused because the physical values of the material constituting the sensor and the object of measurement fluctuate due to change of temperature, because the clearance between the sensor and the object of measurement changes due to difference in the thermal expansion, or the like.
Accordingly, in this apparatus, the torque detection output sometimes changes with change of temperature of the detecting apparatus even if the applied torque is constant, as shown in FIG. 16, thereby making it impossible to obtain a satisfactory detecting accuracy.
As described above, since the above-described torque detecting apparatus have the problems (a) and (b), the torque detecting accuracy cannot be said to be satisfactory.
On the other hand, a sensor which is capable of detecting a torque in a low-rotation region with high response has recently been required in the rotating drive mechanism control system in automobiles, machine tools, etc. Especially, in order to effect the optimum control of an engine, a transmission, etc., a sensor which is capable of detecting a transmitted torque with high response and accuracy in a wide measuring range such as from no rotation to high-speed rotation of an engine and from low-temperature to high-temperature operation has been demanded.
Therefore, it is necessary to solve the above-described problems (a) and (b) as soon as possible.